Conventionally known is a battery monitoring device which monitors an assembled battery including one or a plurality of battery cells in combination and then performs necessary control. With such a battery monitoring device, upon detection of a voltage of each battery cell and determination of an unsafe state such as excessive charging, taking an action such as current interruption can avoid each battery cell from falling into a dangerous state.
Known as circuit configuration for detecting the voltage of each battery cell in the battery monitoring device described above is, for example, a semiconductor circuit as disclosed in PTL 1 below. This semiconductor circuit includes a cell selection circuit formed of a multiplexer provided with a switch in correspondence with each battery cell, and measures, with an AD converter, the voltage of the battery cell selected by use of this cell selection circuit.
Typically, the multiplexer in the semiconductor circuit is composed of a combination of nMOS switches, that is, switches using an n-channel metal oxide film semiconductor transistor. Upon detection of the voltage of the battery cell selected by use of such a multiplexer, to reduce a detection error, it is preferable to ensure a sufficient voltage between a gate and a source of each nMOS switch to thereby suppress ON resistance at a fixed level or below. To this end, a driving voltage which is approximately several voltages higher than the voltage of the battery cell needs to be supplied to each nMOS switch of the multiplexer. In PTL 1, a supply voltage of the cell selection circuit is supplied from the battery cell, at a top, of the plurality of battery cells via an RC filter.
In a case where a load generating high-frequency current noise like an inverter is connected to the assembled battery, a ripple voltage generated by this current noise is superposed on the voltage of the battery cell whereby a detection error occurs in the voltage of the battery cell. In PTL 1, between the semiconductor circuit and each battery cell, a low-pass filter for removing the ripple voltage is provided.